1. Field of the Invention
The field of the invention is that of digital telecommunications, in particular via satellite, and in particular that of digital signal receivers for receiving convolutionally coded signals.
To be more precise, the present invention concerns a device for initializing a Viterbi decoder in a demodulator stage of a receiver for signals transmitted in the form of bursts (TDMA transmission), a receiver comprising an initialization device of this kind and a method used for this initialization.
2. Description of the Prior Art
In the prior art, digital data to be transmitted can be convolutionally coded in the transmitter in order to reduce the error rate on radio transmission between the transmitter and a receiver, for example via a satellite. The convoluted data is decoded in the receiver by a Viterbi decoder. In the remainder of this description, the expression "Viterbi decoder" refers to any decoder using the criterion of maximum likelihood. The decoding is carried out after the received signal is transposed into the baseband and a decision on the symbols is not generally taken until after decoding (this is known as "soft decision").
The use of coding and decoding of this kind for continuous links is known in itself. Reference may be had, for example, to French patent application No. 2 530 096 in the joint names of NEC CORPORATION and KOKUSAI DENSHIN DENWA CO, LTD. concerning a synchronization circuit for a Viterbi decoder using a signal resulting from the difference between the maximum and minimum metrics to obtain synchronization (the mean difference between the maximum and minimum metrics has a high value when synchronization is obtained).
The drawback of this device is that a large number of errors occurs at call set-up time because when the first data is received the Viterbi decoder is not immediately ready to decode it. For this reason a large number of errors is tolerated at call set-up time. This is not a particular problem in the case of continuous transmission, but is unthinkable in the case of TDMA transmission.
In TDMA transmission each burst transmitted comprises a synchronization word, also called the unique word, and message data, the number of message data words and their position relative to the synchronization word being known.
FIG. 1 shows the structure of a transmitted data burst in the case of TDMA transmission.
The burst 1 includes a preamble in the fields CBTR and DUW and message data in the field DATA. The field CBTR includes bits enabling carrier and bit timing recovery. The field DUW includes the symbols of the synchronization word and the field DATA includes the message data bits.
In this burst the synchronization word DUW is in the preamble and its last bit is immediately followed by the first message data bit.
If convolutional coding is to be applied to the message data, the Viterbi decoder in a receiver that receives this burst must be ready to decode message data as soon as its synchronization word is detected, to prevent a burst of errors being produced when the first message data is received. The solution mentioned above (synchronizing the Viterbi decoder by monitoring the divergence of the maximum and minimum metrics) takes too long and cannot be used for TDMA transmission.
The article by Richard L. Moat entitled "ACTS BASEBAND PROCESSING", Globecom '86, IEEE Global Telecommunications Conference, Communications Broadening Technology Horizons, Conference Record CH2298-9, Houston, vol. 1, pages 578-583 describes the provision of a sequence for flushing data written in a Viterbi decoder. This sequence is made up of the first code words received of a sequence, stored in RAM and already decoded.
The drawback of this technique is that it does not prepare the decoder to receive code words and, most importantly, the words received are not necessarily code words since transmission errors may have occurred. In this case the Viterbi decoder can diverge and the last words of the block decoded do not correspond to those of the source.
An object of the present invention is to remedy these drawbacks.
To be more precise, an object of the invention is to provide a device for initializing a Viterbi decoder in a receiver for signals transmitted in the form of bursts, the bursts received possibly coming from different transmitters and being irregularly spaced in time due to the existence of a guard time between the received bursts. The initialization of the decoder must ensure correct decoding of all message data.